San Francisco has big plans for Hunters Point Naval Shipyard, closed in 1974 and being cleansed of contamination by the Navy since 1991, but the city's largest piece of developable land remains a mystery to most.

About 45 curious folks got a close-up look at the deactivated shipyard Saturday on a bus tour offered by the Navy's environmental cleanup program. While the focus of the tour was the $1.1 billion cleanup effort, the group also got a glimpse behind the locked gates and chain-link fences.

During the nearly two-hour tour, they saw already cleansed hillsides, big pits where soil is being tested and fields where it's inspected. They rolled past massive dry docks once used to repair vessels as large as aircraft carriers and a 10-story barracks that was never inhabited.

Tour-goers also got close-in views of the six-story glass periscope tower, the world's largest traveling movable gantry crane, the sound stage where "James and the Giant Peach" was filmed - and a coyote trotting past an air-monitoring station.

San Francisco plans to redevelop the shipyard, long used by the Navy for shipbuilding and repair. Some of the historic structures will stay, but most of the three-story ramshackle wooden buildings will be demolished and replaced with 12,000 new homes and apartments, shopping areas and office and manufacturing buildings, along with hundreds of acres of open space.

But before the city can proceed, the Navy has to clean the 440-acre site of contaminants ranging from radium, arsenic, nickel and manganese to dioxins, PCBs and chlorinated solvent - and it's a big job, the biggest base cleanup in the world, according to the person in charge of the cleanup.

"They call this the billion-dollar base," said Keith Forman, the Navy's environmental coordinator for the shipyard and Saturday's tour guide.

The Navy has spent about $800 million on the cleanup, and expects to spend another $300 million to $400 million before it hands over the final pieces of land to the city in 2021.

"It's just got a lot of contamination," he said. "Much of it is low-level contamination, but it is spread all over the base."

The Navy has already turned over 25 acres to the city, and the first cluster of apartments and condominiums is under construction at the corner of Innes Avenue and Donahue Street. Much more land is expected to be cleaned up and transferred to the city in the next two years.

While the extended cleanup and the development plans have generated controversy in the Bayview-Hunters Point community, there was no animosity or political bickering aboard the bus. The tour, which is held a couple of times a year, was open to the public and attracted mainly neighbors of the sprawling shipyard.

They asked so many questions - ranging from the types of contaminants to the plans for the giant crane (undetermined) - that the tour leaders had to limit them so the bus could keep moving. Tour-goers applauded Forman and said they enjoyed the tour.

"I didn't feel like we got a lot of political spiel," said Christina Pavlov, who lives in the Bayview. "I had expected PR spin. But they answered everybody's questions."

Molly Bloom, who moved to the Bayview three years ago, said she took the tour to satisfy her curiosity about the shipyard.

"It was great learning about what happened here and about what the Navy has been doing," she said. "I'm glad I got a chance to see the base before it's all demolished."